Re: [PATCH 1/2] x86: ioremap: fix wrong physical address handling

[Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>]

(2010/06/17 18:35), Jeremy Fitzhardinge wrote:
> On 06/17/2010 07:03 AM, H. Peter Anvin wrote:
>> On 06/16/2010 09:55 PM, Kenji Kaneshige wrote:
>>
>>>> I think they might be. Kenji?
>>>>
>>> No. My addresses are in the 44-bits range (around fc000000000). So it is
>>> not required for my problem. This change assumes that phys_addr can be
>>> above 44-bits (up to 52-bits (and higher in the future?)).
>>>
>>> By the way, is there linux kernel limit regarding above 44-bits physical
>>> address in x86_32 PAE? For example, pfn above 32-bits is not supported?
>>>
>>>
>
> That's an awkward situation.  I would tend to suggest that you not
> support this type of machine with a 32-bit kernel.  Is it a sparse
> memory system, or is there a device mapped in that range?
>

Device mapped range in my case.
Fortunately, the address is in 44-bits range. I'd like to focus on
making 2^44 work correctly this time.

Thanks,
Kenji Kaneshige




> I guess it would be possible to special-case ioremap to allow the
> creation of such mappings, but I don't know what kind of system-wide
> fallout would happen as a result.  The consequences of something trying
> to extract a pfn from one of those ptes would be
>
>> There are probably places at which PFNs are held in 32-bit numbers,
>> although it would be good to track them down if it isn't too expensive
>> to fix them (i.e. doesn't affect generic code.)
>>
>
> There are many places which hold pfns in 32 bit variables on 32 bit
> systems; the standard type for pfns is "unsigned long", pretty much
> everywhere in the kernel.  It might be worth defining a pfn_t and
> converting usage over to that, but it would be a pervasive change.
>
>> This also affects paravirt systems, i.e. right now Xen has to locate all
>> 32-bit guests below 64 GB, which limits its usefulness.
>>
>
> I don't think the limit is 64GB.  A 32-bit PFN limits us to 2^44, which
> is 16TB.  (32-bit PV Xen guests have another unrelated limit of around
> 160GB physical memory because that as much m2p table will fit into the
> Xen hole in the kernel mapping.)
>
>>> #ifdef CONFIG_X86_PAE
>>> /* 44=32+12, the limit we can fit into an unsigned long pfn */
>>> #define __PHYSICAL_MASK_SHIFT   44
>>> #define __VIRTUAL_MASK_SHIFT    32
>>>
>>> If there is 44-bits physical address limit, I think it's better to use
>>> PHYSICAL_PAGE_MASK for masking physical address, instead of "(phys_addr
>>>
>>>>> PAGE_SHIFT)<<  PAGE_SHIFT)". The PHYSICAL_PAGE_MASK would become
>>>>>
>>> greater value when 44-bits physical address limit is eliminated. And
>>> maybe we need to change phys_addr_valid() returns error if physical
>>> address is above (1<<  __PHYSICAL_MASK_SHIFT)?
>>>
>> The real question is how much we can fix without an unreasonable cost.
>>
>
> I think it would be a pretty large change.  From the Xen's perspective,
> any machine even approximately approaching the 2^44 limit will be
> capable of running Xen guests in hvm mode, so PV isn't really a concern.
>
>      J
>
>